Methods of making inductively heatble articles, induction furnaces and components and materials

ABSTRACT

A method of forming an article comprises the steps of—a) forming an electrically conductive malleable composition to form an uncured shape; and b) inductively heating the shape to cure and harden it and thereby form the article.

This invention relates to inductively heatable articles, inductionfurnaces, components for induction furnaces, inductively heatablematerials, and articles formed from said materials.

Inductive heating can be used in many applications to provide a sourceof heat without the need to connect wiring to the heat generating part.The present invention extends to the use of the materials claimed, inany article formed by induction heating or used in induction heatingapparatus. The materials claimed may also have desirable thermalcharacteristics, and the present invention extends to the use of theclaimed materials however formed.

Induction heating uses eddy currents produced by the interaction of arapidly changing magnetic field with a conductive material. The rapidlychanging magnetic field gives rise to induced currents in the conductivematerial, and these induced currents then produce resistive heating. Theinvention is exemplified in the following description by reference toinduction furnaces, but it will be evident that the invention is notlimited thereto.

Induction furnaces are frequently used in the metal processingindustries to melt materials. When a material to be melted issufficiently electrically conductive, that material can be directlyheated through induction. ‘Rammed’, electrically non-conductive,refractory linings are typically used for metal containment in suchapplications. Alternatively, pre-formed clay-bonded SiC/graphitecrucibles are frequently used. However, some materials do not “suscept”(interact with the electromagnetic field) very well, and among these arealuminium. For such materials heating has to be by an indirect route andtypically this is by providing an electrically conductive crucible. Asan example pre-formed carbon/silicon carbide based crucibles are used insuch applications.

A problem with this approach is that it restricts the range of sizes andshapes of the furnace, and also inhibits the manufacture of largefurnaces, since large crucibles are both difficult and expensive tomake.

A further problem in such an arrangement, is that if a crucible cracks,the molten metal within the crucible can escape and damage the furnace.Accordingly when a crucible cracks it needs to be replaced completelywith consequent cost.

The present invention provides a malleable composition that may have aresistivity low enough to effectively couple with the induction field ofan induction furnace, and in doing so, be cured. The composition cancontain a self-glazing additive to provide any necessary oxidationresistance. The malleable composition can be used to form a liner to aninduction furnace in situ. It can also be used to repair cracks in suchliners.

The composition also has improved thermal characteristics such that itcan be used in conventional forming processes to advantage.

By malleable is meant that the material is sufficiently deformable andadherent that it is capable of being fashioned into shape by hammeringor pressure. The material may be in the form of an adherent powder thatadheres under pressure. Any method that is used for forming malleablematerials can be adopted—(for example and without limitation-pressing,ramming, and rolling).

The scope of the invention is made apparent from the claims in the lightof the following illustrative description with reference to thedrawings, in which:—

FIG. 1 is a schematic circuit diagram of an induction furnace;

FIG. 2 is a plot of frequency versus resistivity for a crucible asspecified below;

FIG. 3 is a diagram showing the forming of an induction furnaceaccording to the invention;

FIG. 4 shows a sheathed thermocouple according to the invention; and

FIG. 5 shows the results of thermal response tests of the sheathedthermocouple of FIG. 4 and a more traditional sheathed thermocouple.

INDUCTION FURNACE THEORETICAL CONSIDERATION

Eddy currents are made use of in induction furnaces. A schematic circuitdiagram of a typical induction furnace layout is given in FIG. 1.Typically a source of medium frequency alternating current 1 suppliescurrent to a water cooled coil 2 surrounding the crucible 3 to beheated. The circuit has a power factor correction capacitance 4.

The rapidly changing magnetic field of the coil induces EMFs which giverise to induced currents in those parts of the crucible and its contentswhich are conducting.

The measure of the ability of a coil to give rise to a back emf is knownas the self inductance of the coil. It is defined by: $\begin{matrix}{E = {{- L}\frac{\mathbb{d}I}{\mathbb{d}t}}} & (1)\end{matrix}$

The induction will offer an opposition to the current flow due to theback emf and try to impede the changes which are producing it (Lenz'sLaw). This impedance is called the inductive reactance, X_(L), which isgiven by:X_(L)=ωL  (2)

A capacitor in an AC circuit is continuously being charged anddischarged. Increasing the frequency of the supply increases the rate atwhich the capacitor is charged and discharged and therefore, increasesthe reactive current. The applied voltage lags the current by π/2. Thusthe impedance which a capacitor offers to current flow is called itsreactance, X_(C), and is given by: $\begin{matrix}{X_{C} = \frac{1}{\varpi\quad C}} & (3)\end{matrix}$

The effective resistance which the circuit shown in FIG. 1, as a wholeoffers to current flow is called the impedance, Z, and is defined by:Z=√{square root over (R ² +(X _(L) +X _(C) ) ² )}  (4)

Both X_(L) and X_(C) depend on frequency and the frequency which causesthe current to be maximum is called the resonant frequency and occurswhen X_(L)=X_(C). $\begin{matrix}{f_{o} = \frac{1}{2\quad\pi\sqrt{LC}}} & (5)\end{matrix}$

The penetration depth of the eddy current is dependent upon both theresistivity of the material and the frequency of operation.$\begin{matrix}{{{Penetration}\quad{{Depth}({cm})}} = \sqrt{\frac{\rho}{4\quad\pi\quad\omega\quad\mu}}} & (6)\end{matrix}$Where:ρ=resistivity of the material, ×10⁹ω=2πfμ=permeability ˜1.

Typical commercial induction furnaces operate at frequencies in therange 50 Hz to 10,000 Hz although higher frequencies are achievable.Ideally the crucible wall thickness should be greater than thepenetration depth in order to couple efficiently within the cruciblewall. The properties of a typical crucible for an induction furnace(e.g. an Excel™ crucible obtainable from Morganite Crucible Limited,Norton, England) are shown below: Resistivity (Ωcm) 0.005 Crucible WallThickness. 4 cm Operating Frequency (Hz) 10,000

Taking the operating frequency at 10,000 Hz then the typical penetrationdepth for an Excel® crucible is calculated to be 3.55 cm. According toequation (6) the higher the resistivity of the material is, the greaterthe penetration, or the higher the required operating frequency, inorder to couple within the crucible wall. The frequency can be increasedby reducing the capacitance as shown in equation (5), i.e. byincorporating a variable capacitor.

However, reducing the capacitance will reduce the power factor. Thepower factor (pf) is defined as the ratio between real power (kW) andthe total power supplied (kVA). Total power is made up of two componentscalled Real Power (real work done) and reactive power (serves no realfunction).

Reducing the capacitance will increase the reactive component of powerand hence reduce the power factor (pf).

Thus, if an electrically conductive malleable composition is to provideeffective coupling with the induction field, the only options are toprovide a low resistivity electrically conductive malleable compositionsuch that it will couple at normal operating frequencies, or to increasethe frequency to allow for a higher resistivity electrically conductivemalleable composition.

FIG. 2 shows a plot of frequency required to couple at a depth of 3.72cm for a 4 cm wall thickness crucible versus the resistivity of thematerial. As is demonstrated in the plot the greater the resistivity ofthe material, the greater the frequency necessary to couple within thecrucible wall.

Therefore, for an electrically conductive malleable composition layer of4 cm thickness to effectively couple with normal operating frequenciesof 50 Hz to 10,000 Hz the resistivity of the electrically conductivemalleable composition would need to be below about 0.0055 Ωcm. To coupleat a typical frequency of about 3,000 Hz the resistivity of theelectrically conductive malleable composition would need to be belowabout 0.002 Ωcm. This of course assumes that a malleable compositionwould have to be applied to a similar thickness as a crucible wallthickness. If thinner thicknesses are applied either the resistivitywould have to be lower, or the operating frequency higher. Conversely, athicker layer implies that a higher resistivity can be tolerated orlower frequencies used.

Electrically Conductive Malleable Material Requirements

In addition to a requirement for electrical conductivity, theelectrically conductive malleable material has other requirements.

In use, crucibles manufactured from graphite and silicon carbide can beexpected to hold molten substrates at temperatures as high as 1400° C.,or in some cases higher, therefore a number of physical properties arerequired of them. These properties include flexural strength, thermalconductivity, oxidation resistance and erosion resistance.

Electrically conductive silicon carbide based crucibles aretraditionally formed from a mixture of silicon carbide powder andgraphite flakes bound together by the carbonised residue of a bindercompound, for example a resin, pitch or tar. The manufacturing stepstypically comprise several of the following steps:—

-   -   pressing the mixture of silicon carbide, graphite, and binder to        form a green body    -   “fettling” the green body (e.g. machining the body to a final        green shape, adding spouts or handling lugs)    -   curing the green body to remove volatiles from the binder and/or        set the binder    -   firing the green body at a temperature and for a time sufficient        to carbonise the binder    -   applying a glaze to the finished crucible to protect the body of        the crucible against oxidation

Typically, the pressing step is by either isostatic pressing or byroller pressing (in which a roller presses the mixture against theinside of a mould).

Before firing, the binder holds together the “green” crucible to provideadequate mechanical strength for the handling and fettling. Once curedand fired, the binder carbonises to leave a residual carbon skeletonthat contributes to the structure of the crucible.

The use of carbon precursors based on resin, pitch and tar in themanufacture of crucibles is coming under increasing pressure due toenvironmental, health and safety concerns. In the past, legislationassociated with these matters has been a factor in the replacement ofpitch and tar with phenol based resins such as novalac resins. There arenow increasing health concerns with the use of phenol based binders, andlegislation may eventually make their use uneconomical.

In use, the glaze applied to the crucible can be damaged throughmechanical abuse, and such damage exposes the core of the carbon/siliconcarbide crucible to attack (primarily through oxidation).

The above problems in the manufacture of crucibles are amplified whenone is considering an electrically conductive malleable composition thathas to be installed and fired in situ. Desirably the electricallyconductive malleable composition, to provide performance comparable to afired crucible, should:—

-   -   have a high thermal conductivity, when cured, to avoid “hot        spots”    -   show oxidation resistance when cured    -   resist erosion when cured.

In addition, to make best use of its malleable nature, the electricallyconductive malleable composition desirably should:—

-   -   minimise the amount of noxious vapours released    -   provide only minimal quantities of vapour on curing so as to        reduce the risk of cracking or spalling    -   not require a separate glazing step to provide oxidation        resistance    -   be capable of “self healing” so that damage to the glaze is        repaired without specific attention        Reduction of Noxious Vapour

Existing resin, pitch, or tar based binders would produce unacceptablequantities of noxious vapours. The applicants have realised that waterbased binders would be preferable, since these will minimise or nullifythe generation of hydrocarbons during curing and firing. Several waterbased binders are possible, including sugars. Indeed the use of dextrineto provide some binding in the unfired state and provide carbon as abinder on firing is contemplated. However, the applicants have foundthat a water based carbon dispersion (for example a graphite dispersion)provides good binding activity to produce a coherent body, whilst notgenerating any hydrocarbons during firing.

Because the carbon is provided in a water based dispersed form it ofnecessity has a fine particle size and so has a high surface activity.The high surface activity means that the particles of carbon readilybind to the coarser particles of the material (e.g. graphite flakes andsilicon carbide) and so act as a binder. A typical particle size ofcarbon in the dispersion is <5 μm, and preferably <2 μm to get goodbinding through electrostatic attraction, although colloidal sizedparticles (<1 μm) would provide higher surface activity andelectrostatic attraction.

In tests, the applicants used two water based graphite dispersion(Metaflo 4000™, a water based graphite dispersion available from RocolLimited of Leeds, England, normally used as a lubricant for hot metalworking tools and a specially prepared graphite dispersion from PilamecLtd, Unit 40/41, Lydney Industrial Estate, Lydney, Gloustershire, GL154EJ) having the properties set out below.

In tests, the applicants have also found that the physical green bindingstrength which is an important criteria for the malleability of the mixis influenced by the viscosity of the binder which in turn is influencedby the graphite content. Metaflo 4000 ™ Pilamec Graphite content ˜21%˜30% Particle size ˜50% <2 μm ˜50% <8 μm Viscosity at 25° C. 2.5 mPA · s15.7 mPA · s Specific gravity 1.13 1.2

Such water based binder systems can be used generally for binding carbonand carbon/silicon carbide materials, so avoiding the use of morenoxious components.

Green Binding

Green binding is an important criteria of the mix, as the mix shouldmaintain its pre-formed shaped once rammed or pressed into any artefactwithout distortion or slumping. In addition to the use of high graphitecontent dispersed binder, in tests, the applicants have found that theaddition of one or more clays—e.g. high absorbent/pliable clays such asbentonite, and/or borax (e.g. a colloidal borax solution), which wouldnot evolve noxious fumes during heating, improved the green strength ofthe mix and improved adhesion and pliability of the mix. However, theapplicant have also found that the addition of these clays increases theresistivity of the mix such that the amount had to be kept to a minimum.Typically additions of less than 2% were used.

Prevention of Cracking and Spalling

A water based binder will still release water on curing of theelectrically conductive malleable composition, and that water couldcause cracking or spalling. This is particularly so if heating is rapidas the water will all form steam at 100° C.

The applicants decided to use superabsorbers as an additive.Superabsorbers are very powerful hygroscopic polymeric materials,commonly used in baby's nappies and other absorbent sanitary towels (seefor example WO9415651, WO9701003, and US2001047060). Superabsorbers areconventionally used in such applications as granulated materials or aswoven or non-woven textiles.

When added as a fine powder to the electrically conductive malleablecomposition, typically at less than the 1% level, the applicants havefound superabsorbers absorb water from the composition, and release itat a range of higher temperatures from 100° C. upwards. The materialused by the applicants in tests was a sodium/potassium polyacrylate,which is a non-toxic white powder. The material was bought in bulk underthe trade name Supersorp® from Huvec Klimaatbeheersing of Postbus 5426,3299 ZG MAASDAM, Belgium.

Typically, superabsorbers are provided as a granulate. The powder usedby the applicants was a fine powder with 75% between 75-150 μm.Preferred materials have 75% by weight or more of a size less than 150μm.

Superabsorbers such as sodium polyacrylate are polymeric materialshaving a large number of hydrophilic groups that can bond with water.The present invention extends to any hygroscopic polymeric material,such as a superabsorber, that can absorb large quantities of water andrelease the water over a range of temperatures. Typically asuperabsorber can absorb more than 5 grams of water per gram of materialand absorbencies of >10 g/g, >15 g/g and >20 g/g are not unusual (seeU.S. Pat. No. 5,610,220) and indeed absorbencies of >100 g/g are knownfor distilled water of 400-500 g/g and lower in salt solutions (e.g.30-70 g/g in 0.9% NaCl solution). Preferred materials for the presentinvention have absorbencies for distilled water above 100 g/g, morepreferably above 200 g/g.

Further applications of superabsorbers to drying refractory materialsare set out in co-pending International Patent Application WO03/106371.

Self-Glazing and Self-Healing

The applicants decided that some self-glazing property would beadvisable. Self-glazing is known for some ceramics. Typically a glass ora flux is included in the material so that on firing it can form a skinover the ceramic. Self glazing has rarely been used for carbon/siliconcarbide materials in the past. Use of a glass or flux is howevercompatible with such materials. In particular, the applicants have foundthat incorporation of boron containing materials in a conventionalcrucible mix provides such self-glazing properties.

The applicants believe that the boron containing materials oxidise toform B₂O₃, which reacts with any other glass formers present to form aglaze. A particularly useful form of boron containing material is boroncarbide, which gives the best results the applicants have found to date.Other boron containing materials which give a self glazing effectinclude boron nitride. Boron carbide is used as an anti-oxidant forrefractory materials, as is boron nitride, but its use to form a glazeis unreported.

Because the material of the glaze is part of the electrically conductivemalleable composition, damage to the glazed surface is healed throughcontact of the unglazed body with air.

Lowering Resistivity

As is discussed more fully below, lowering the resistivity of theelectrically conductive malleable composition can be achieved by severalmeans. These include the use of exfoliated graphite flakes, which have ahigh surface area and/or carbon fibre. In a typical green mix,electrical conductivity is provided by current passing from particle toparticle within the mix. If the particles of the mix are of higherconductivity than any continuous phase (as is typically the case) thenthe bulk of the resistivity is accounted for by the need for current tojump from particle to particle.

An exfoliated graphite provides a high surface area so that current canbe collected from and transferred to a large number of other conductiveparticles. This can reduce the number of particle/particle junctionsthat current has to cross and so reduce resistivity.

In similar fashion, carbon fibres can transfer current over longdistances compared with the particle size of a typical green mix.

EXAMPLES

A series of compositions were made having the compositions set out inTable 2 below based upon a base mix set out in Table 1 below. 5 kg ofmix was mixed in a Z-blade mixer for 20 minutes. The mix was then rammedinto an alumina crucible. The alumina crucible was then placed in atraditional induction furnace with an operating frequency of 3000 Hz.TABLE 1 Raw material Wt % Specification Graphite flake 34.1 >84% C,90% >180 μm, 10% <500 μm Silicon carbide 38.1 >95% SiC, 50-70% 180-355μm Alumina coarse 5.3 >94% Al₂O₃, 65-90% 250-355 μm Alumina fines0.5 >60% Al₂O₃, 90% <75 μm Ferrosilicon powder 5.7 72-80% Si, 65% <53 μmSilicon powder 6.0 >97% Si, 65% <53 μm Borax 4.1 80% <75 μm Supersorp ®0.4 See description above Boron carbide 3.8 >95% B₄C, 95% <53 μmDextrine 2.0 See description above +water based carbon +15.0 Metaflo4000 ™ - see description above binder

Due to the low conductivity of the base mix (given in Table 1 anddesignated Mix A in Table 2), the lining did not suscept sufficient tocure. Instead the lining was used to melt cast iron at 1500° C. It wasproposed that the heat from the metal would allow the mix to heat up andhence, self glaze.

Various methods were used to improve the conductivity of the green mix.These include the use of exfoliated graphite flakes, which have a highsurface area and carbon fibre. Two types of exfoliated graphites wereused, TimCal Graphite BNB90 with a surface area of 26.02 m²/g andSuperior Graphite EX21 with a surface area of 21.68 m²/g. Around 5% ofthe graphite flakes was added to a standard mix as shown in Table 2.TABLE 2 Base Mix A B C D E F G H I +EX21 5 5 5 +BNB90 5 5 5 5 5 +3 mmfibres 0.05 0.1 0.1 +6 mm fibres 0.05 0.1 0.1 Resistivity Ωcm 0.143 0.060.133 0.02 0.02 0.017 0.01 0.018 0.016

Another approach to improving the conductivity was the addition ofcarbon fibres. Two sizes of carbon fibres (Graphil™ 34-700) were used, 3mm and 6 mm in length and added in 0.05% and 0.1% quantities. The fibreswere dispersed in the mix using a coarse sieve prior to the addition ofthe water based carbon binder.

Investigative mixes were pressed into bars of dimension 153 mm×26 mm×15mm and density 2.1 g/cm³. Due to the fragile nature of the pressed bars,which made electrical resistivity measurements difficult, the bars werecured to 150° C. to provide some handling strength prior to measurement.Resistivities were measured and the results are summarised at the footof Table 2.

The addition of exfoliated graphite flakes had a marked impact to theconductivity of the base Mix A recipe (compare for example theresistivities of Mix A and Mix B). This in combination with the carbonfibres shown for Mix D to G further improves the conductivity. The morefibres in the mix the better the conductivity. A final value ofresistivity of 0.01 Ωcm was achieved. According to the plot of frequencyversus resistivity shown in FIG. 2, this would mean that the mix wouldcouple at a frequency of 18 kHz for a 4 cm crucible wall thickness. Thefrequency is within an acceptable range, however, further improvementsto the conductivity are still possible by using more conductive carbonfibres based on pitch/tar.

Example 2

500 kg of mix I shown in Table 2 above was mixed in a high shear MortonMixer. The mixing procedure is shown below.

FIG. 3 shows a furnace arrangement in which furnace 5 comprisesinduction coils 6, cooling coils 7, slip plane 8, alumina protectionblanket 9, rammed induction lining 10, and base lining 12. The mix wasinitially rammed using an air pressure hammer on the floor of theinduction furnace to create a base lining 12 approximately 5 cm thick. Acardboard support cylinder 11 of diameter 30 cm was then placed on theinside of the induction furnace which was of diameter ˜48 cm and ˜71 cmdeep. To provide extra protection to both the cooling coils 7 andinduction coils 6 a ˜2.5 cm thick alumina fibre blanket 9 was placedagainst the insulating slip-plane 8 covering the cooling coils 7. Themix was rammed using an air pressure hammer in the ˜5 cm gap between thecardboard support cylinder 11 and the alumina blanket 9 covering theslip-plane 8. A rammed lining 10 of about ˜5 cm thick was created usingthis technique. With an operating frequency of 1900 Hz and power in therange 80 kW-100 kW, the mix was inductively heated to 1500° C. to bothcure and fire the mix. The pre-fired lining was then used to melt ironat 1500° C.

Example 3

In this application, the applicant took advantage of the malleability ofthe mix to isostatically press various foundry artefacts, for examplethermocouple sheaths for non-ferrous applications. Unlike the inductivelining for such an application there is no real requirement forelectrical conduction as the thermocouple sheath will not be heated byinduction but merely thermal conduction. For this reason there were nocarbon fibre additions. As thermal conduction plays a major part in theproperties of the sheath, the presence of the exfoliated graphite wasimportant.

12 kg of the base mix shown in Table 1 but using as water based binderPilamec™ instead of Rocol™, and using 5% of an exfoliated graphitepowder ABG1025 from Superior Graphite with a surface area of 18 m²/g asthe exfoliated graphite additive, was mixed in a small plough shearmixer (miniature version of the Morton Mixer) using the mixing procedureshown below.

FIG. 4 shows a schematic diagram of a sheathed thermocouple comprising asteel former 13, thermocouple 14 and isostatically pressed sheath 15.The mix was isostatically pressed around the hollow steel former 13,(21.5 mm diameter and 463 mm long), to a pressure of ˜13.8 MPa [2000PSI] to create a sheath 15 of 44 mm outer diameter.

The sheath was then cured to 150° C. for 2 hours to provide some greenstrength. The whole assembly was finally fired to 1025° C. to activatethe self-glazing properties of the mix.

Thermal response tests in molten aluminium of the sheathed thermocouple,and of a similar thermocouple made from a more traditional crucible mixwere conducted. The tests consisted of preheating the sheathedthermocouples to and plunging them into molten aluminium at ˜700° C. Thetests showed [see FIG. 5] that the rammable mix gave a quicker responsetime (line A) than the traditional crucible mix (line B), reaching thetemperature of the melt sooner despite being preheated to a lowertemperature. This indicates a significantly higher thermal conductivity.

Alternative Curing Methods

Providing a malleable composition that has an adequate electricalconductivity in the green state to cure completely of itself ispossible. However it can prove advantageous, particularly where themalleable composition is insufficiently conductive in the green state tocouple efficiently, to place an electrically conductive former (e.g. asteel shell) inside the lined furnace and heat this shell by induction.This can act to indirectly heat and cure the malleable composition. Inthe curing process the conductivity will rise, so that the cured liningwill couple better than in the green.

In addition to usage in traditional induction furnaces, the associatedmalleable characteristics with low resistivity will also permit the mixto be used in a range of other applications where induction heating is arequirement. For example the material may be used to form inductivelyheatable tube furnaces for treating material passing through the tubes.

The material results in articles having an improved susceptibility incomparison with conventional materials. The “Q” value of an article isdefined as the ratio of the total power KVA to the real power KW orconversely the inverse of the power factor, e.g. for a work pieceresulting in a “Q” of 5, to generate 100 KW in the work piece you wouldneed a total power of 500 KVA in the work coil.

Hence the lower the “Q” value, the lower the reactive power required inthe coils to generate the same power in the work piece. Commercialinduction crucibles result in a “Q” value of around 10, whereas the “Q”value of a crucible made with the materials of the present invention hasbeen shown to be around 5.

Crucibles according to the present invention are thus more efficientthan a conventional crucible. The same applies to other inductivelyheated articles.

Equally the malleable characteristics of the mix will allow the mix tobe pre-formed into any artefacts such as crucibles or thermocouplesheaths via a range of pressing techniques typically isostatic pressingor uniaxial pressing.

The thermal characteristics of the material provided by various carbonand carbide components would also allow the pre-formed artefacts to befired by any other means such as gas or electric firing. In the latter,oxidation resistance can be provided by either the traditional glazingroute or self glazing.

1. A method of forming an article comprising the steps of:— (i) formingan electrically conductive malleable composition to form an uncuredshape; and (ii) inductively heating the shape to cure and harden it andthereby form the article.
 2. A method, as claimed in claim 1, in whichthe article is an inductively heatable part of a heating apparatus.
 3. Amethod, as claimed in claim 1, in which the inductively heatable part isthe lining to an induction furnace.
 4. A method, as claimed in claim 1,in which the electrically conductive malleable composition has aresistivity of less than 0.04 Ω·cm.
 5. A method, as claimed in claim 3,in which the electrically conductive malleable composition has aresistivity of less than 0.02 Ω·cm.
 6. A method, as claimed in claim 1,in which the electrically conductive malleable composition includes asan ingredient graphite flakes.
 7. A method, as claimed in claim 6, inwhich the amount of graphite flakes is greater than 20% by dry weight ofthe electrically conductive malleable composition.
 8. A method, asclaimed in claim 7, in which the amount of graphite flakes is greaterthan 30% by dry weight of the electrically conductive malleablecomposition.
 9. A method, as claimed in claim 6, in which the flakegraphite is or includes an exfoliated flake graphite.
 10. A method, asclaimed in claim 1, in which the electrically conductive malleablecomposition includes as an ingredient silicon carbide.
 11. A method, asclaimed in claim 10, in which the amount of silicon carbide is greaterthan 20% by dry weight of the electrically conductive malleablecomposition.
 12. A method, as claimed in claim 11, in which the amountof silicon carbide is greater than 30% by dry weight of the electricallyconductive malleable composition.
 13. A method, as claimed in claim 1,in which the electrically conductive malleable composition includes asan ingredient a water based carbon dispersion binder.
 14. A method, asclaimed in claim 13, in which the water based carbon dispersion binderis or includes a graphite.
 15. A method, as claimed in claim 14, inwhich the graphite is a colloidal graphite.
 16. A method, as claimed inclaim 13, in which the carbon provided by the water based carbondispersion binder is present in amount less than 20% by dry weight ofthe electrically conductive malleable composition.
 17. A method, asclaimed in claim 1, in which the electrically conductive malleablecomposition includes as an ingredient carbon fibres.
 18. A method, asclaimed in claim 1, in which the electrically conductive malleablecomposition includes as an ingredient a hygroscopic polymeric materialcapable of retaining water in the mixture over a range of temperaturesabove the boiling point of water.
 19. A method, as claimed in claim 18,in which the hygroscopic polymeric material has an absorbency of morethan 5 grams of water per gram of material.
 20. A method, as claimed inclaim 19, in which the hygroscopic polymeric material has an absorbencyof more than 10 grams of water per gram of material.
 21. A method, asclaimed in claim 20, in which the hygroscopic polymeric material has anabsorbency of more than 100 grams of water per gram of material.
 22. Amethod, as claimed in claim 21, in which the hygroscopic polymericmaterial has an absorbency of more than 200 grams of water per gram ofmaterial.
 23. A method, as claimed in claim 18, in which the hygroscopicpolymeric material is a polyacrylate.
 24. A method, as claimed in claim18, in which the hygroscopic polymeric material comprises a fine powderwith 75% by weight or more of a size less than 150 μm.
 25. A method, asclaimed in claim 1, in which the electrically conductive malleablecomposition includes as an ingredient a self-glazing constituent.
 26. Amethod, as claimed in claim 25, in which the self-glazing constituent isor includes a boron containing material.
 27. A method, as claimed inclaim 26, in which the self-glazing constituent is or includes boroncarbide.
 28. A method, as claimed in claim 1, in which the electricallyconductive malleable composition is a rammable composition and is rammedinto a former to form the article.
 29. A method, as claimed in claim 1,in which step b) comprises at least in part indirect heating by aninductively heated former.
 30. A malleable composition comprising in dryweight percent of the composition:— graphite flakes>20% siliconcarbide>20% and further comprising a water based carbon dispersionbinder.
 31. A malleable composition, as claimed in claim 30, in whichthe water based carbon dispersion binder is or includes a graphite. 32.A malleable composition, as claimed in claim 31, in which the graphiteis a colloidal graphite.
 33. A malleable composition, as claimed inclaim 30, in which the carbon provided by the water based carbondispersion binder is present in amount less than 20% by dry weight ofthe malleable composition.
 34. A malleable composition, as claimed inclaim 30, in which the amount of graphite flakes is greater than 30% bydry weight of the malleable composition.
 35. A malleable composition, asclaimed in claim 30, in which the flake graphite is or includes anexfoliated flake graphite.
 36. A malleable composition, as claimed inclaim 30, in which the malleable composition includes as an ingredientcarbon fibres.
 37. A malleable composition, as claimed in claim 30, inwhich the malleable composition includes as an ingredient a hygroscopicpolymeric material capable of retaining water in the mixture over arange of temperatures above the boiling point of water.
 38. A malleablecomposition, as claimed in claim 30, in which the malleable compositionincludes as an ingredient a self-glazing constituent.
 39. A malleablecomposition, as claimed in claim 38, in which the self-glazingconstituent is or includes a boron containing material.
 40. A malleablecomposition, as claimed in claim 39, in which the self-glazingconstituent is or includes boron carbide.
 41. A malleable composition,as claimed in claim 30, in which the malleable composition comprises agreen binding additive.
 42. A malleable composition, as claimed in claim41, in which the green binding additive comprises one or more clays. 43.A malleable composition, as claimed in claim 41, in which the greenbinding additive comprises borax.
 44. An uncured shape formed from themalleable composition of claim 30 to
 43. 45. An article formed by curinga shape as claimed in claim
 44. 46. An article as claimed in claim 45,in which the article is an inductively heatable part of a heatingapparatus.
 47. An induction furnace lined with an inductively cured andhardened electrically conductive malleable composition formed by themethod of claim
 1. 48. A method of binding carbon or carbon/siliconcarbide materials comprising the use of a water based carbon dispersionbinder.
 49. A method, as claimed in claim 48, in which the water basedcarbon dispersion binder is or includes a graphite.
 50. A method, asclaimed in claim 49, in which the graphite is a colloidal graphite.